The kinematics of z≳6 quasar host galaxies

Abstract

We explore the kinematics of 27 z ≳ 6 quasar host galaxies observed in [C II] 158 μm ([C II]) emission with the Atacama Large Millimeter/submillimeter Array at a resolution of ≈0 25. We find that nine of the galaxies show disturbed [C II] emission, due to either a close companion galaxy or a recent merger. Ten galaxies have smooth velocity gradients consistent with the emission arising from a gaseous disk. The remaining eight quasar host galaxies show no velocity gradient, suggesting that the gas in these systems is dispersion dominated. All galaxies show high velocity dispersions with a mean of 129 ± 10 km s−1. To provide an estimate of the dynamical mass within twice the half-light radius of the quasar host galaxy, we model the kinematics of the [C II] emission line using our publicly available kinematic fitting code, qubefit. This results in a mean dynamical mass of 5.0 ± 0.8(± 3.5) × 1010 Me. Comparison between the dynamical mass and the mass of the supermassive black hole reveals that the sample falls above the locally derived bulge mass–black hole mass relation at 2.4σ significance. This result is robust even if we account for the large systematic uncertainties. Using several different estimators for the molecular mass, we estimate a gas mass fraction of >10%, indicating that gas makes up a large fraction of the baryonic mass of z ≳ 6 quasar host galaxies. Finally, we speculate that the large variety in [C II] kinematics is an indication that gas accretion onto z ≳ 6 supermassive black holes is not caused by a single precipitating factor.